With the growing number of users and demand of bandwidth, traditional macro cell can barely meet performance requirements. To help address this, 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) networks introduced a low power node (LPN) cell, such as a picocell, femtocell, etc. A picocell covers a small cellular area, such as buildings (offices, shopping malls, train stations, stock exchanges, etc.), aircraft, etc. Picocells are typically used to extend coverage to indoor areas where outdoor signals do not reach well. Picocells may also add network capacity in areas with very dense phone usage, e.g., train stations, public squares, etc. Femtocells are typically designed for use in a home or small business. Femtocells differ from picocells in that they are intended to be much more autonomous. For example, femtocells tend to be self-installed by the end user in their home or office, primarily for their own benefit.
Such LPNs enable LTE network topology, e.g., a heterogeneous network (HetNet), to become more flexible. A HetNet comprises macro cells overlaid with femtocells, picocells and other LPN cells, which can satisfy the operators burgeoning traffic demands through cell-splitting gains obtained by bringing UE closer to their access points. Employing HetNet, however, presents numerous challenges. For example, user equipments (UEs) in a HetNet, especially those UEs at a cell edge, receive two or more intra-frequency signals simultaneously. If the received signals are strong, the UEs will suffer severe inter-cell interference when receiving the serving cell signal. Such interference will degrade the user experience.
In HetNet, the number of UEs connected to a picocell is much smaller than the number connected to a macrocell, which results in inefficient resource utilization. Therefore, it is beneficial for a network to bias handover preferentially towards picocells. Such a bias, e.g., may be achieved by adding an offset to the picocell Reference Signal Received Power (RSRP) so that UEs can select a picocell even if it is not the strongest cell. This technology is referred to as a Cell Range Extension (CRE). Due to the preferential biasing, CRE may increase the radio range of a picocell, and may enable a macrocell to offload more UEs to a picocell.
However, the introduction of CREs may further aggravate the inter-cell interference problem. For example, a picocell signal received by the UEs in the CRE zone may have heavy interference from one or more aggressor cells (e.g., a femtocell, a macrocell, etc.) because the aggressor cell signal is still strong, and perhaps is even stronger than the picocell signal.
To solve the intra-frequency interference problem in HetNet, the 3GPP Release 10 specification introduced Enhanced Inter-Cell Interference Coordination (eICIC). eICIC redefines an additional time dimension, which allows the signals from different cells to be orthogonal in the time domain. The use of this additional time dimension is referred to as Almost Blank Subframe (ABS). ABS only includes the necessary signals and channels for compatibility with UEs of 3GPP Release 8/9, e.g., the Primary Synchronization Signal/Secondary Synchronization Signal (PSS/SSS), Physical Broadcast Channel (PBCH), Cell-specific Reference Signal (CRS), System Information Block 1 (SIB1), etc. eICIC configures the ABS in the aggressor cell, and these ABSs are used by the serving cell to provide service for the UEs served by the serving cell that previously experienced strong interference from the aggressor cell(s). In this way, the inter-cell interference problems can be well managed.
As noted above, the ABS broadcasts the PBCH. The PBCH carries a Master Information Block (MIB), and consists of a limited number of the most frequently transmitted parameters essential for the UE to gain initial access to the cell. For example, the MIB may carry four pieces of important system information: the transmit antenna port number, the system bandwidth, the System Frame Number (SFN), and the Physical Hybrid Automatic Repeat reQuest (ARQ) Indicator Channel (PHICH) indicator. The PBCH is designed for early detection by the UE, and cell-wide coverage. When the cells are synchronized at the frame level, however, PBCH(s) transmitted by aggressor cell(s) will overlap the serving cell PBCH. As a result, aggressor cell PBCH(s) will interfere with the serving cell PBCH. For example, when the UEs are all located in a CRE zone, the interference can be 9 dB stronger than the wanted signals (worst case), which will degrade the PBCH performance severely.
If the UE cancels the aggressor cell PBCH(s) from the received PBCH, the UE can obtain an interference free representation of the serving cell PBCH. The UE can use such an interference free serving cell PBCH to improve the detection of the PBCH. Therefore, PBCH Interference Cancellation (PBCH-IC) is desirable for HetNet.